The present invention relates to fluid flow control and, more particularly, to a uniquely configured servovalve system having a fail safe apparatus incorporated therein for switching the servovalve system between an operating position and a fail safe position upon occurrence of a failure of the servovalve system.
Servovalves are often utilized in a fluid system to precisely manipulate or regulate the flow rate and/or pressure of fluid flowing within a fluid circuit. The fluid, which can include both liquids and gases, is typically employed to move an actuator, which is conventionally comprised of a piston sealed within a cylinder. The servovalve manipulates the fluid flowing within the fluid circuit in order to move the piston by forcing fluid into one end of the cylinder while simultaneously withdrawing or exhausting the fluid out of an opposing end of the cylinder. Servovalves are most often used in closed-loop fluid systems wherein the position of the actuator, and velocity and/or pressure of fluid flowing within the fluid circuit is continuously monitored with a feedback device which generates system feedback signals.
A controller uses the system feedback signals to generate command signals that are received by the servovalve. The servovalve responds to the command signals to regulate the fluid flowing within the fluid circuit in such a manner so as to minimize the error between a desired position of the piston and an actual position of the piston within the cylinder. Servovalves generally incorporate a spool which either rotates or slides axially in a housing to port the fluid flow to a desired location. Utilizing hydraulic amplification of the command signal, some servovalves utilize fluid pressure acting on a small pilot stage to provide motive force to position the spool.
However, direct drive servovalves position or drive the spool directly using an electrically powered stepper motor that is mechanically connected to the spool. Stepper motors are comprised of multiple magnetic detents that incrementally oppose rotation of the stepper motor when power is applied. However, even when power is not applied to the stepper motor, the magnetic detents continue to oppose rotation of the stepper motor. Unfortunately, this characteristic of stepper motors presents a danger to property and personnel in that the spool will remain in its last position upon loss of power to the stepper motor. Therefore, the fluidic device under control, such the above-mentioned actuator, will be in an unknown position when power is restored to the stepper motor.
For this reason, it is highly desirable to provide a fail safe mode in the event of a failure. Failure of the fluid system may include a loss of fluid pressure within the fluid circuit and/or a loss of electrical power to the servovalve and/or failure of the controller. In the fail safe mode, the piston is preferably moved to one of its stroke extremes by allowing pressurized fluid to enter one end of the cylinder while simultaneously allowing fluid to escape an opposing end of the cylinder. Because the failure of the fluid system may include a loss of electrical power, it is necessary that the servovalve system may be placeable in the fail safe position even when the servovalve system is rendered inoperable.
In prior art fluid systems, a fail safe apparatus may be provided by incorporating redundant servovalves or a network of solenoid valves into the fluid circuit. Unfortunately, such additional components add to the complexity, cost and maintenance of the fluid system. In addition, such additional components add to the overall volumetric requirements for the fluid system. Volumetric requirements are particularly important for very large servovalves typically used with high flow rate fluid systems due to the relatively large-sized components that are required. Because most fluid systems require a fail safe system having similarly high flow rates, the addition of the fail safe system essentially doubles the volumetric requirements.
As can be seen, there exists a need in the art for a direct drive servovalve system having a fail safe apparatus for switching the servovalve system to a fail safe position upon occurrence of a variety of failure conditions including, but not limited to, loss of electrical power and failure of the controller. Furthermore, there exists a need in the art for a direct drive servovalve system having a fail safe apparatus that is of simple construction, of low cost, and which is easily maintained. In addition, there exists a need in the art for a direct drive servovalve system having a fail safe apparatus that is relatively compact in order to limit the overall volumetric requirements for the fluid system.